Luminance is an optical property that allows a large number of photons to pass, per unit time, through a small aperture within a small range of angles. In casual parlance, luminance is often referred to as “brightness”. In engineering applications not directly concerned with the spectral sensitivity of the human eye, it is most straightforwardly measured in units of watts (of power) per square mm (of output aperture) per steradian (of solid angle): W/(mm2 sr).
The optical power from a source of high luminance can be focussed down to a tiny spot or projected (“thrown”) great distance as a collimated beam. The so-called optical Liouville theorem, also known as the “optical brightness theorem”, also known as the “preservation of étendue”, demands that luminance is a conserved quantity through a loss-less system of geometric optics comprising lenses and mirrors.
One might, naively, consider luminous intensity, that is the optical power per unit area of aperture, in units of W/mm2 (watts per square millimetre), as a useful performance parameter. But luminous intensity, unlike luminance, is not preserved through an optical system of lenses and mirrors.
Bright sources of light enable many lucrative applications. In addition to luminance, the qualities of a light source that go to determine whether it is adequate/advantageous for a particular application include:                its spectral profile as characterized, in the case of a single emission line, by its vacuum wavelength and linewidth, both in units of nm (nanometres) or, more generally by its output spectrum in units of W/nm (watts per nanometre);        its total output power (luminous intensity integrated across its whole output aperture) in units of W (watts); and        its wall-plug efficiency (optical power out divided by electrical power) in dimensionless units.        
Certain optical applications—e.g. CD/DVD recording (“burning”), welding/cutting, long-distance fibre and free-space telecommunication, optical ranging/sighting and projected energy weaponry—require light sources of high luminance, i.e. in the ball park of 103 W/(mm2 sr). Other applications—e.g. general room/street lighting, indicator lights, visual display panels—can suffice with luminances a million times smaller, i.e. around 10−3 W/(mm2 sr). The two values of luminescence stated here are intended only to supply a rough, quantitative sense of scale to clarify what may be meant by “high” and “low” luminance. And other applications—e.g. video projection, skin treatment and hair removal—work with what might be described as “intermediate” levels of luminance: around 1 W/(mm2 sr).
Many familiar sources of light—such as lamps and LEDs—though powerful, do not generate light of sufficient luminance to enable high- or even intermediate-luminance applications. Sometimes these applications can be made to work at a low duty cycle by operating lamps in pulsed mode (e.g. a xenon flash lamp) or by temporarily over-driving LEDs (beyond their maximum CW operating current). But such methods have drawbacks: xenon flash lamps deteriorate with every flash (life times of 10,000 flashes are typical); and LEDs can only be over-driven by factors of a few before failing.
Stimulated emission, harnessed in devices known as lasers, is capable of generating light of extremely high luminance, both in pulses and continuously. Often, it is the only viable way of doing so. In consequence, the word/epithet “laser” has become prefixed to the names of many different applications, e.g. “laser eye surgery” and “laser ranging”, that require high-luminance light. But lasers have certain drawbacks of their own: a finite pump threshold (so difficult/impossible to “simmer” at low output power), often low overall (“wall plug”) energy conversion efficiency, high manufacturing cost, mechanical fragility, sensitivity to temperature changes, limited operational lifetime, and a need for skilled operators (regular re-alignment of mirrors, replacement of consumed/degraded materials). In particular, there are still certain colours of light that cannot be generated efficiently using lasers: these include cyan (around 510 nm) and from lemon yellow through to orange (550 nm to 610 nm).
Luminescent materials comprise lumophores arranged within what is otherwise an optically transparent medium. Spontaneous luminescent decay combined with total internal reflection, harnessed in devices known as luminescence (or fluorescence) concentrators (“LCs”), can generate light at luminances of a factor of 10-1000 times greater than what their pump lamps or pump LEDs can provide directly. In other words, LCs can generate up to around 10 W/(mm2 sr) which is greater than the luminance of sunlight. Because the fluorophores or phosphors used within LCs do not need to be suitable for lasing, there is a wider choice available. As a result, LCs can be identified that output efficiently at those wavelengths lasers output inefficiently; where the difference in efficiency can exceed a factor of 10.
FIG. 1 depicts, in the 2-D space 100 spanned by output power and luminance, areas where lasers 101, lamps 105, LEDs and luminescent concentrators 103 (pumps by lamps) occupy zones of advantageous with respect to viability, manufacturing and operating costs.
LCs are advantageous in applications requiring (i) high optical powers, where wall-plug efficiency becomes a major thermal/financial consideration, but where only (ii) intermediate levels of luminance and needed, and where (iii) the spectral purity/coherence provided by lasers is also not required. Examples of applications that lie in this niche include: optical skin treatments (e.g. hair removal, spider vein removal), photo-chemical processing, optical pump sources for lasers and masers, and video projections systems.
Luminescent concentrators have been developed to direct solar energy onto photovoltaic cells. To date, they have not however been widely adopted due to poor efficiency: too much solar energy gets dissipated inside the concentrator due to “self-absorption” where light emitted from a fluorophore/phosphor gets absorbed by another fluorophore/phosphor. Self absorption has also hampered the development of efficient light sources incorporating luminescent concentrators.
FIG. 2, for example, shows the absorption 201 and fluorescence spectra 203 of Rhodamine 6G. Self-absorption is typified by the overlap between the absorption 201 and fluorescence spectra 203.
A problem is that the black absorption and red fluorescence curves overlap. The “Stokes' shift” is the difference in optical wavelength between the absorption peak and the fluorescence peak. The problem of self-absorption has been addressed in recent decades by the discovering of “high-Stokes shift” materials that suffer less self-absorption. These include organic dye molecules, inorganic quantum dots, and inorganic oxides doped with rare earths (often similar/related to those used as laser crystals/glasses). In particular, cerium-doped YAG provides a system where the absorption and fluorescence overlap very little. The absorption peak of cerium-doped YAG lies at 460 nm (and is narrow), whereas the emission peak lies at 540 nm.
Luminescent concentrators have recently been considered as light sources in U.S. Pat. Nos. 7,898,665 and 7,208,007. These patents use cylindrical or hexagonal rods of Stokes shift material. The present disclosure relates to providing a further improved light source based on a luminescent concentrator using relatively high Stokes shift materials. The present disclosure also provides an advantageous device for coupling light out of the improved light source.